Hydraulic brake systems used on motor vehicles have one or more master cylinders that direct hydraulic fluid under pressure to linear actuators mounted on vehicle wheel assemblies. Linear actuators expand to apply drum-type brakes or to apply disk-type brakes. The linear actuators contract to release the brakes. In a properly functioning hydraulic brake system, depressing the brake pedal forces hydraulic fluid into linear actuators and expands the actuators. The brake shoes or brake pads of each wheel assembly are forced, by an actuator, into contact with a brake drum or a brake rotor disk. Continued movement of the brake pedal increases the braking force supplied. Substantially full braking force is applied with movement of the brake pedal through a portion of its full stroke in a properly operating system. The hydraulic brake system is able to apply sufficient braking force with a small movement of the brake pedal because the hydraulic brake fluid is incompressible.
Occasionally, air finds it way into a hydraulic brake system. If the master cylinder becomes low on hydraulic brake fluid, for example, air can enter the system through the master cylinder. Air can also enter the system if a line is loosened. The build up of air in a hydraulic brake system can prevent the system from applying the required force on the brakes, due to the compressibility of air. Manual movement of the brake pedal compresses the air in the hydraulic brake system, rather than expanding the linear actuator, and the brake pedal may become fully depressed before sufficient pressure is produced in the system to fully engage the brakes. Air in a hydraulic system may expand, due to an increase in temperature. The pressure generated by the thermal expansion of the air can expand the linear actuator and cause a brake to drag when the brake is not being applied by the vehicle operator. Brake drag generates additional heat, increases the temperature of the air and may eventually result in brake failure.
Linear actuators have been equipped with air bleed valves for removing air from a brake system, since hydraulic brakes were first introduced. These valves allow brake fluid to be pumped through the lines to force air in the lines, and air in the linear actuators, out through an open bleed valve. The air bleed valves, when working properly, purge air from a hydraulic system. Linear actuators for drum-type brakes are mounted on a brake mounting plate attached to an axle housing or a steering knuckle assembly. The bleed valve is exposed on the surface of the brake mounting plate facing the wheel on the opposite side of the vehicle. Linear actuators for disk brakes are part of a caliper supported adjacent to a disk rotor. The wheels on an automotive vehicle normally, at least partially, enclose the linear actuator. The air bleed valves are, however, exposed to spray, dirt, dust and other contaminants from road surfaces in about the dirtiest location on a vehicle and to the inside of the wheels, where they are generally not seen and are not cleaned. Thus, the air bleed valves that are currently in use are susceptible to damage by corrosion. The valves tend to corrode due to contaminants that contact the threads on the brake bleed valve and fill a large chamber adjacent to the valve seat. Manufacturers of motor vehicles have, in recent years, placed a snap-on plastic cap over the discharge passage of air bleed valves. The plastic caps are frequently knocked off and lost. It also appears that the plastic caps allow contaminants to seep into the discharge passage over time. Once contaminants enter the discharge passage, the plastic cap tends to retain them in the discharge passage.
The threads on brake bleed valves are exposed to these corrosive agents on the outside of the linear actuator at a point where the air bleed valve screws into the linear actuator. The threads on air bleed valves are also exposed to thee corrosive agents in a discharge passage inside the linear actuator. Over a period of time, the threads on the air bleed valves and the threads inside the linear actuators that receive the air bleed valves become corroded to the point that it is not possible to turn an air bleed valve and open a discharge passage to allow air to be purged from the system. Corrosion can also damage the valve seats.
Advanced corroding of air bleed valves generally makes it necessary to replace the entire linear actuator. This is clearly an expensive procedure. In some cases, a linear actuator can be removed from a vehicle, the piston or pistons and seals can be removed and then the linear actuator housing can be heated and the air bleed valve removed. The time required to disassemble and remove an air bleed valve that is frozen due to corrosion can be substantial. It may, therefore, be less expensive to replace the old linear actuator with a complete new unit than to remove a frozen brake bleed valve.
The air bleed valve corrosion problem has been recognized for many years. Numerous solutions have been attempted. Air bleed valves have been made from stainless steel, for example. Stainless steel bleed valves are difficult to machine and are expensive. The threads in the linear actuator, which receive the stainless steel air bleed valve, still corrode. Multipart complicated valve arrangements which reduce corrosion problems have been designed. These complicated valves are expensive and are often usable only in linear actuators that are designed to receive them and may not be useable as replacement bleed valves in linear actuators employed on vehicles in use at this time.